1. Field of the Invention
This invention relates to processes in which a catalyst powder is suspended in a liquid.
2. Description of the Prior Art
In a slurry reactor, for example, one in which a mixture of hydrogen and carbon monoxide is reacted on a powdered catalyst to form liquid hydrocarbons and waxes, the slurry is maintained at a constant level by continuously or intermittently removing wax from the reactor. The catalyst in the wax must be separated from the slurry and returned to the reactor to maintain a constant inventory of catalyst in the reactor. In order to keep the catalyst losses within the replacement rate due to deactivation, the wax removed from the system must not contain more than about 0.5% catalyst by weight.
Several devices have been proposed for separating the catalyst from the wax including centrifuges, cross-flow sintered metal filters, wire mesh filters, and magnetic separators. Centrifuges are unable to reduce the catalyst concentration below about 1% and are complex, costly, and difficult to maintain. Sintered metal and wire mesh filters have been found to irreversibly plug. Magnetic filters typically can not process fluids with greater than about 0.5% solids.
U.S. Pat. No. 6,068,760, which is incorporated into this document by reference, describes a dynamic settler for separating catalyst from the reactor slurry. The dynamic settler provides several advantages over other separation methods including: (i) it does not require backwashing, (ii) it operates continuously, (iii) it does not require costly filter media, (iv) it is relatively simple and cost effective and (v) it can not plug. However, for plants that produce wax at a rate greater than about 0.25 gpm, the size of the settler must be increased to the point where natural convection begins to have a negative effect.
Natural convection is driven by buoyancy forces that arise due to temperature differences. The parameter that relates this driving force to the viscous retarding force is the Grashof number, which is proportional to diameter cubed. Thus, increasing the settler diameter dramatically increases the effect of natural convection. Tests in large vessels, six to fourteen feet in diameter with Fischer Tropsch slurries have shown that it is not possible to separate the catalyst and molten wax by settling. The solution to this problem has been to use many small settlers in parallel which can quickly become impractical.
An object of the invention is to provide an improved apparatus for separating wax and catalyst whereby relatively clean wax can be removed from the slurry reactor and the catalyst can be returned to the reactor without being subjected to attrition from a mechanical pump.
Another object is to prevent natural convection flows in large-scale dynamic settlers.
Other objects will become apparent as the description of the invention proceeds.
With this invention, a portion of a slurry containing wax and catalyst is passed from a reactor to a dynamic settler, which defines a closed chamber. A vertical feed conduit extends downwardly into the chamber for a substantial distance, forming an annular region between the inner walls of the chamber and the feed conduit. A slurry removal outlet at the bottom of the settler chamber returns slurry back to the reactor. As the slurry flows through the settler, the heavier catalyst particles settle out and are removed as the slurry at the bottom of the settler is recycled back to the reactor. Clarified wax rises up in the annular section and is removed by a wax outlet pipe at the top.
According to this invention, the annular region within the settler is substantially filled with a baffle that defines a great number of parallel channels. By making the cross-section of each channel sufficiently small, one minimizes natural convection flow which would tend to keep the catalyst particles suspended in the wax.